One way of measuring and examining the well-functioning and performance of a patient's heart is by determining the cardiac output. A well-established method or principle to measure the cardiac output is by inert gas rebreathing, which is a non-invasive measurement method. This principle utilizes a closed rebreathing system where a small amount of a blood soluble, physiologically inert gas is inhaled from a rebreathing bag. Among other things, a concentration curve of the inert gas can be extracted from the measurements during rebreathing and the wash-out rate, which is proportional to the cardiac output, can be calculated. During the measuring, the patient is breathing through a respiration valve which allows switching from breathing air to rebreathing the inert gas mixture from the bag and switching back again. A similar respiration valve is used in other cardiopulmonary tests such as in determination of the lung volume or the diffusing capacity of the lung, where the ability of the lung membrane to transport gases is determined. In general it can be used in rebreathing tests and in single breath or multiple breath wash out tests.
Different types of both manually, mechanically, and pneumatically operated valves have been used to date for cardiopulmonary testing. One type of mechanically operated valve used in respiration valves is a manually operated sliding or rotating type of valve where the flow is shut off by a moving inner part. Also metal flap valves opened and closed manually or by pneumatics have been used. Yet another type of valve that has been used is the inflatable balloon, in which a small balloon-like element is placed centrally in the flow channel and blown up to stop the flow. However, all of these types of valves in a respiration valve yield several problems and disadvantages. Firstly, any parts intruding into the flow are to be avoided as they increase the flow resistance and thereby affect the patient's breathing. Secondly, any metal parts make the valve far more clumsy, heavy, and take up a considerable amount of space. All of these points are very important in the design of a respiration valve, which is to be connected to a patient's mouth for a certain period of time and therefore must be small and light. Also if possible, metal parts are to be avoided in an item as a respiration valve which is in close contact with different humans (or animals), their moisture and saliva, both for sanitation reasons and for mechanical reasons as the metal deals poorly with such environmental conditions. Furthermore, a mechanically operated valve is sensitive to wear and fatigue during many opening and closing operations.
A pneumatic pinch valve can in its simplest form consist of a cylindrical flexible tube sealed in each end to a substantially rigid co-axial valve body carrying the tube, where the flexible tube, when subjected to a fluidic force from the surrounding fluid (e.g. air), is pinched together and closes. However, such a cylinder will inevitably form foldings and wrinkle when pressed together. A valve like this will not be able to close tightly unless the pressure in the surrounding fluid is relatively high, and furthermore, as the shape and the area of the opening are uncontrollable, the valve will be unsuitable for regulation purposes.
The pneumatic pinch valve described in CA 2299913 is a circular flexible tube in a rigid valve body where the flexible inner tube collapses along a lateral constriction seam hereby avoiding the undesired wrinkles mentioned above. In order to ensure that the tube will actually collapse in the desired fashion, a repetitive series of conditioning pre-manipulations is performed on the tube. This means that the cylinder is subjected to a substantial and somewhat permanent deformation giving the cylinder wall a crease at the desired positions. An alternative mentioned in the patent is to reduce the wall thickness at the desired bending points or lines. This valve construction contains the advantage of being very simple and with the possibility of being very compact and small. Nevertheless, some big problems still remain. First, although the wrinkling is avoided, a valve as described above still cannot close completely in the corners at the ends of the constriction seam due to the shape of the cylinder unless the pressure in the surrounding fluid is relatively high. Second, the steps taken to ensure that the tube is pinched together along a linear seam instead of the uncontrollable folding have inevitably damaged the tube to some extent, thereby reducing the lifetime of the valve and also reducing the resistance to wear and fatigue considerably.
Another pneumatic pinch valve is described in the patent WO 0017549, where a flexible diaphragm in a tube is compressed by air pressure. In the open condition the diaphragm is circular and in the closed condition the membrane folds and is pinched together around a set of guiding vanes, one in each side inside the membrane. This valve has solved the aforementioned problem with incomplete and uncontrollable closing and can be advantageous in large scale structures such as in ventilation ducting and in conveyors for powder transport. The guiding vanes inside the membrane, however, lead to more parts in the valve which complicates the structure and the manufacturing hereof, also because the guiding vanes are of a rather complicated geometrical shape. This valve is therefore more expensive to produce and cannot be made as compact or small, the larger dimensions again also leading to the need for a higher air pressure to close the valve. Furthermore, the guiding vanes inside the tube are undesirable in many applications as they increase the flow resistance considerably. Also the pinch shape of this valve makes the membrane walls pass a snap-through under increasing pressure from the open to the closed position. During such a snap-through the open flow area changes very fast and suddenly leading to difficulties in adjusting and regulating the flow. The valve is thus not so adequate for regulation purposes.
U.S. Pat. No. 5,119,825 describes a single-use, disposable patient valve for the use in cardiopulmonary tests. Here, the patient breathes through a mouthpiece on the other side of which is placed a first valve. If activated, the patient is forced to hold his breath. The mouthpiece is then connected to three channels, one of which is connected to a gas supply regulated by a demand valve. The second channel leads to a collection tube and the third leads to atmospheric air, which passage can also be opened or closed by a valve. Both valves work by a mechanically operated piston pinching off the circular flexible channels. Although avoiding any moving parts protruding into the flow, the proposed respiration valve possess a number of disadvantages. The construction of the valves themselves yields a heavy and clumsy design leading to a quite heavy and big apparatus which is inappropriate in the testing situation of patient, both for a patient at rest, but even more so for measurements made on a patient during different physical tests. A further disadvantage is the mechanical moving parts which although they are not in direct contact with the breathing of the patient remain a hygienic problem. Also, the design of the respiration valve with the placing of the pinching valves is disadvantageous as the dead space between the two valves is considerable rendering the measurements correspondingly inaccurate.